LITTLE ROCK — The U.S. Supreme Court blocked Arkansas from beginning a series of executions Monday. But the state has vowed to press ahead with plans to execute several inmates before the end of April, when its supply of a key lethal injection expires.

Arkansas' death penalty protocol includes two drugs that are typically used in surgery and one that benefits cardiac patients.

A look at the drugs:

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MIDAZOLAM

Midazolam is the key drug in the procedure and one that Arkansas hasn't yet used in an execution. It would be the first of the three drugs administered and would be used to sedate the inmate. Because the state's supply reaches its expiration date April 30, Arkansas scheduled eight executions before then.

At normal adult doses of around 4 mg for routine surgery, midazolam can slow or stop breathing to the point that medical literature advises doctors to monitor patients closely. With a 500 mg dose listed in the state's execution protocol, Arkansas expects that the inmates will not be aware they are dying.

In federal court testimony last week, doctors differed on whether midazolam is an appropriate execution drug, though the U.S. Supreme Court has ruled in 2015 that it is.

U.S. District Judge Kristine Baker had said Saturday that Arkansas' execution protocol doesn't outline what would happen if the inmate were to remain conscious even if given a double dose. She also said the protocol doesn't lay out what executioners intend to do to ensure that the inmates are unconscious. The 8th U.S. Circuit Court of Appeals on Monday reversed her decision.

In typical medical settings, midazolam relaxes people and produces amnesia.

The second drug, vecuronium bromide, is a muscle relaxant, but not in the typical sense. Rather than being prescribed for tightness or muscle pain, vecuronium is used to prevent muscles from moving so they don't interfere with surgeons. After receiving the drug, patients must be on a ventilator or they will suffocate because their diaphragm cannot move.

One doctor testified that being given vecuronium bromide without sedation and ventilation would be equivalent to being held underwater. The typical dose is up to .1 mg/kg intravenously, or 8.5 mg for the typical inmate set to die this month. Under Arkansas' protocol, executioners would administer 100 mg, or more than 11 times the typical dose, five minutes after the midazolam is administered and once the inmate is unconscious.

The AP last year used redacted drug labels to identify Hospira, which was purchased by Pfizer, as the likely manufacturer of Arkansas' vecuronium bromide. The supplier McKesson Corp. has said it sold Arkansas the drug for use in inmate health care, not executions.

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POTASSIUM CHLORIDE

Potassium is essential for maintaining a proper heart rhythm, and levels that are too high or too low can cause heart trouble. Medical experts who testified before Baker said that the final drug, potassium chloride, causes considerable pain when injected and is typically diluted when given to patients.

A typical dose is 20 milliequivalents delivered over several hours. Arkansas plans to use 240 mEq immediately after the inmate is injected with vecuronium bromide.

The director of the Arkansas Department of Correction, Wendy Kelley, testified last week that Arkansas was not charged for its current supply of potassium chloride. She said the supplier wished to remain anonymous and did not want to generate a bill of sale so there would be a smaller chance of being identified.

Fresenius Kabi USA, a subsidiary of the German company Fresenius Kabi, said last week that it appeared to have manufactured the potassium chloride the state plans to use.

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Follow Kelly P. Kissel on Twitter at https://twitter.com/kisselAP and go to http://bigstory.ap.org/author/kelly-p-kissel to see his work.